Frequency multiplier



as g

Dec. 10, 1957 A. HAHNEL FREQUENCY MULTIPLIER Filed Nov. 4,1953

OUTPUT 0 l FIG.|

Fi nf FIG.2

INVENTOR.

ALWIN HAHNEL (wakjfi-Zuzncv A T TOR/VEY United States Patent 2,816,227 FREQUENCY MULTIPLIER Alwin Hahncl, Little Silver, N. J., assignor to the United States of America as represented by the Secretary of the Army Application November 4, 1953, Serial No. 396,265

6 Claims. (Cl. 250-36) (Granted under Title 35, U. S. Code 1952), sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment of any royalty thereon.

This invention relates to frequency spectrum generators and more particularly to frequency multipliers wherein a high degree of multiplication is desired at a prescribed frequency.

The usual procedure :in frequency multiplication by means of electron discharge devices comprising several electrodes consists in designing the electron discharge device as an amplifier and :in tuning its output to a frequency nf which is a harmonic of the excitation frequency 1. These different harmonics vary very greatly in amplitude, being generally of :much greater amplitude at the lower harmonic frequencies than at the higher harmonic frequencies. Hence the multiplication systems used heretofore required a large number of stages to provide any extended factor of multiplication. In such systems, a high degree of frequency accuracy is difficult to obtain if a multiplicity of frequency channels is required under conditions which do not allow use of a separate crystal for each one. Moreover, the isolation of the desired harmonic frequencies from the many spurious oscillations and unwanted harmonics becomes very complicated and is difficult to achieve.

It is therefore an object of the present invention to provide a frequency spectrum generator in which the selection of a high order multiplication factor is readily accomplished by tuning of a single resonance circuit.

It is another object of the present invention to provide a frequency spectrum generator utilizing a single vacuum discharge tube.

Still another object of the invention is to provide a combined oscillator-multiplier system which has improved performance characteristics.

In accordance with the present invention, there is .provided a frequency spectrum generator comprising a vacuum tube having at least a plate, a grid and -a cathode. Two discrete resonant means are included in circuit with the plate for producing simultaneous excitation at a predetermined fundamental frequency and at a preselected frequency substantially equal to a' desired multiple of the fundamental frequency. Also included are means in circuit with the grid for generating the fundamental frequency whereby thepreselected frequency is keyed such that its phase is periodic at the fundamental frequency to produce the desired output multiple frequency.

For a better understanding of the invention, together With other and further objects thereof, reference is had 2,816,22 Patented Dec. 10, 1957 ice to the following description taken in connection with the accompanying drawing in which:

Figure 1 schematically illustrates the frequency spec trum generator, and

Figures 2 and 3 show a group of explanatory curves.

Referring now to Figure 1 of the drawing, the frequency spectrum generator shown therein comprises an electron discharge device 10, having an anode 12, a cathode 14 and a control grid 16. Control grid 16 is connected to one terminal of piezoelectric crystal 18 through frequency choke coil 2-0, the other end of piezoelectric crystal 18 being grounded at '19. Crystal 18 is by-passed to cathode 14 by means of resistor 22, and cathode 14 is connected to ground through choke coil 23. It is to be understood of course that any other suitable gridleak arrangement may be used. Anode 12 is connected to B+ through serially connected choke .coil 24 and a resonant circuit 25 which comprises the parallel arrangement of capacitor 26 and inductance 28. The values of capacitor 26 and inductance 28 are such that resonant circuit 25 is tuned to resonate substantially at the frequency generated by piezoelectric crystal 18, hereinafter referred to as the fundamental frequency h. The piezoelectric crystal 1% and resonant circuit 25 may be considered to be a tuned plate-tuned grid oscillator adapted to oscillate at a frequency f which, as explained hereinbelow, is the fundamental frequency common to all available channel frequencies. A conventional output resonant circuit 30, herein shown as a butterfly type resonator, is connected between anode 12 and grid 16, one terminal of resonant circuit 30 being connected to anode 12 and the other terminal of resonant circuit 30 being coupled to grid 16 through blocking capacitor 32. The value of blocking capacitor 32 should be such that the impedance thereof is high for the fundamental frequency f Regenerative feedback at both frequencies from plate 12 to grid 16 is provided in the same manner as in conventional oscillators. Resonant circuit 30 is tuned to a preselected frequency F close to or approximately equal to nf symbolically noted as F fth, where nf is a desired output multiple f of the fundamental frequency generated by piezoelectric crystal 18. To distinguish between preselected frequency F and the desired multiple frequency f =nf f will hereinafter be referred to as the output frequency. The circuit shown in Figure 1 provides for simultaneous self-excitation of oscillations at two independent frequencies in a single'tube. The fundamental requirement for such simultaneous oscillations in a one-tube stage is that there must be a relatively large frequency ratio between the two frequencies. The value of choke coil 24 should be so chosen that the shunt impedance is very high. The series combination of choke coil 20 and crystal 18 in the grid circuit maintains grid 16 above ground for the preselected frequency F. Cathode choke 23, of course, functions as a conventional isolating choke for radio frequencies. Resonant circuit 30 is conventionally coupled to an output circuit 36 which, as explained hereinbelow, provides an output of only the desired output frequency f =nf In considering the operation of the spectrum generator, reference is made to Figures 2 and 3. Curve A of Figure 2 represent the fundamental frequency f of oscillations generated by piezoelectric crystal 18. The

frequency f is arbitrarily chosen to provide the fundamental frequency to which any one of the selected output frequencies f derived from output circuit 36 is harmonically related. As stated above, the piezoelectric crystal 1S and resonant circuit provide for oscillations at the fundamental frequency h which is generated in tube 10 simultaneously with the preselected frequency F. The voltages of both the preselected frequency F and the fundamental frequency f are illustrated respectively in curves A and B of Figure 2. The grid voltage shown in curve A of Figure 2 may be considered to provide a bias such that there is provided a regenerative period and a degenerative period for the preselected frequency F. The regenerative period of the preselected frequency F occurs during the positive half-cycle of the fundamental frequency oscillation f and the prescribed frequency F oscillations increase during this regenerative period until a non-linear range of the tube characteristic becomes involved and a constant amplitude condition is reached. The degenerative interval occurs for the negative half-cycle of the fundamental oscillation h where the prescribed frequency oscillations F are quenched due to the decrease in transconductance of the oscillator tube 10 resulting from the negative half-cycle of frequency h. This results in a positive damping of the prescribed frequency F oscillations. As explained hereinbelow the oscillations at prescribed frequency F from tuned circuit must start with the same phase at the beginning of each regenerative period so that, in effect, the preselected frequency F is keyed and phase controlled such that its output wave form is periodic at the frequency f which is the fundamental frequency common to all available channel frequencies. To obtain a crystal stabilized output frequency f =nf the resonant a circuit 30 should be tuned close to the desired output frequency. With frequency F in the vicinity of the desired output multiple frequency f =nf the output energy from the higher frequency tank circuit 30 is concentrated at the desired output multiple frequency. As shown in Figure 3 there appears a frequency spectrum wherein the amplitude of the desired multiple output frequency f =nf is a maximum only if preselected frequency F is in the close vicinity of nf The frequency F does not appear in the output circuit 36 inasmuch as this preselected frequency is periodically phase controlled at the fundamental frequency f By such an arrangement, the output frequency from circuit 30 is an exact harmonic, or multiple, of the fundamental frequency. This is a well known principle and it is believed that no further description is necessary. Of course, if it is desired to shift the multiple output frequency, then the resonant circuit 30 must be tuned to a prescribed frequency F in the vicinity of the newly desired multiple frequency output. Thus, the spectrum generator may put out any desired one of a very large number of crystal controlled frequencies. If continuous tuning over a wide frequency range is provided, the circuit merely functions as though it were electronically detented.

As the amplitude of the preselected oscillation increases, grid current is drawn during a considerable portion of the regenerative period. As a result the mean potential at grid 16 becomes increasingly negative thus reducing the negative resistance of the circuit, and thereby the rate of increase of the oscillation amplitude, until an equilibrium amplitude is reached. This is clearly shown in curve B and is represented by the duration T It is to be understood, of course, that although preferable, it is not essential to reach a state of equilibrium. The equilibrium amplitude will be maintained until the beginning of the degenerative phase where the oscillations at frequency F decay exponentially as represented by the duration T This is so because the negative voltage portion of the sine wave of curve A reduces the transconductance of tube 10 below the value at which th higher frequency circuit is oscillatory. Thus the frequency F is quenched at the crystal controlled fundamental frequency such that during the degenerative period T the oscillations decay. The time constant of the build-up function may be shortened by conventionally increasing the feedback. To shorten the decay time, a low circuit Q is required. In order to prevent the output from being that'of a carrier which is amplitude-modulated at the fundamental frequency h, it is necessary that the oscillations F be periodic at this fundamental frequency. This requirement is most easily fulfilled if the oscillations F are made to disappear in the noise level before the arrival of the next keying pulse as shown. With the circuit shown in Figure 1, it has been found that the unwanted adjacent harmonics may be suppressed up to 46 db below the desired harmonic frequency output f =nf by tuning exactly to the desired multiple frequency output.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A frequency spectrum generator comprising a vacuum tube having at least a plate, a grid and a cathode, means in circuit with said grid for generating radio frequency energy at a fundamental frequency, two discrete resonant means in circuit with the plate, one of said resonant means being tuned to said fundamental frequency and the other of said resonant means being tuned to a prescribed multiple of said fundamental frequency whereby there are produced respective simultaneous self-excited oscillations at said fundamental frequency and at said multiple frequency, said fundamental frequency generating means including means for periodically generating said multiple frequency oscillations at said fundamental frequency such that the initial cycle of each of said periodic multiple frequency oscillations have the same relative phase.

2. The spectrum generator defined in claim 1 wherein said discrete resonant means each comprise parallel resonant circuits.

3. A frequency spectrum generator comprising a vacuurn tube having at least a plate, grid, and a cathode, a first resonant circuit in circuit only with the plate and adapted to oscillate at a fundamental frequency, a second resonant circuit in circuit with the plate and the grid and adapted to oscillate at a preselected frequency substantially equal to a desired multiple of the fundamental frequency, said first and second resonant circuits being simultaneously self-excited, means in circuit with the grid for generating radio-frequency energy at the fundamental frequency, said last mentioned means including means whereby the preselected frequency oscillation is periodically keyed at the fundamental frequency such that the initial cycle of each of said periodic multiple frequency oscillations have the same relative phase to produce a frequency spectrum wherein only the desired multiple frequency is at maximum amplitude.

4. A frequency spectrum generator comprising a vacuum tube having at least a plate, a grid and a cathode, means in circuit with said grid for generating radio frequency energy at a fundamental frequency, two discrete resonant means in circuit with the plate, one of said resonant means being tuned to said fundamental frequency and the other of said resonant means being tuned to a prescribed multiple of said fundamental frequency whereby there are produced respective simultaneous selfexcited oscillations at said fundamental frequency and at said multiple frequency, said fundamental frequency generating means including means for periodically generating said multiple frequency oscillation for only the positive half-cycle of said fundamental frequency, the initial cycle of each of said periodic multiple frequency oscillations having the same relative phase.

5. The spectrum generator defined in claim 1 wherein said fundamental frequency generating means comprises a piezoelectric crystal.

6. The spectrum generator defined in claim 1 wherein said multiple frequency resonant means comprises a parallel resonant circuit in circuit with said plate and said grid.

References Cited in the file of this patent UNITED STATES PATENTS Gerth Apr. 15, 1930 Osnos Sept. 10, 1935 Klotz Nov. 3, 1936 Hershberger July 25, 1944 Grisdale Feb. 27, 1951 Christensen et al. Nov. 10, 1953 FOREIGN PATENTS Germany Apr. 22, 1954' 

